252 Conceptual Design

Bridge design is a combination of art-creation, science-natural laws, and technology-engineering reality. The conceptual design is the first step, which the designers must to take, to visualize and imagine the bridge in order to determine its fundamental function and performance before any theoretical analysis and detailing design can be proceeded. The design process includes considerations of important factors, such as selection of bridge systems, materials, proportions, dimensions, foundations, esthetics, and surrounding landscape and environment.

25.2.1 Planning — Fulfillment of its Function

The first principle is planning a bridge to optimally meet the specified functions. A bridge project will start with planning the fundamental design conditions. To meet the specific purposes, a bridge may have different alignments: straight, skewed, and horizontally curved (Figure 25.3). A straight bridge is easy to design and construct but often needs longer spans. A skewed bridge or a horizontally curved bridge is commonly required for expressways or railroads where the road line must be kept straight or horizontally curved, even at the cost of a more difficult design. The width of a bridge is dependent of the traffic requirements. For a highway bridge, its width is usually determined by the width of the traffic lanes and the sidewalk width, and often the same dimension as that of the approaching road.

25.2.2 Bridge Types — Challenging Spans

The type of bridges is usually determined by factors such as design loads, surrounding geographical features, soil and foundations, passing line and its width, the length and span of the bridge, esthetics, the requirement for clearance below the bridge, transportation of the construction materials, erection procedures, construction cost and period. Table 25.1 shows the span lengths appropriate to each type of bridge.

25.2.3 Proportioning — Golden Mean and Image of Human Body

The geometry proportioning of a bridge structure is to establish the relationships between horizontal superstructures and supporting substructures, between the depth and span of the beams, between the height, length, and width of the opening, and between sizes of individual members. The ideal bridge is structurally straightforward and elegant. The golden mean provides the key to squaring the circle. The image of a beautiful human body provides the harmonious proportion — all measurements and their relationship are found all numerical relationships (Leonhardt 2000).

FIGURE 25.3 Bridge lines.

TABLE 25.1 Types of Bridges and Applicable Span Length

FIGURE 25.3 Bridge lines.

TABLE 25.1 Types of Bridges and Applicable Span Length

Bridge type

Span range (m)

Leading bridge and span length

Prestressed concrete girder

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